Hi All.

Someone asked me about possible solutions for an over-dutied control panel. This particular panel basically controls a furnace. It is 480V and 500A rated. The control panel manufacturer supplied very detailed drawings of the panel. It has multiple breaker, terminal blocks, contactors etc. Fairly typical. The manufacturer clearly states on the drawings that the panel has an SCCR of 5kA. Many of the individual components on the drawings have their withstand or interrupting ratings shown. Some components are 5kA, including the panel's main disconnect switch, other are 10kA, 65kA etc.

A short circuit study for the facility was done. The calculated available 3 phase symmetrical bolted fault current at the line side of the control panel is about 32kA.

My first suggestion was to contact the panel manufacturer to see if they could achieve a series rating with J fuses. My next thought was to look at various types of 500A Class J and Class T fuses and use the up-over-down method on the peak let-through curves to see if there was a possibility of going that route. All the curves I looked at for 32kA available still had a peak let through of well more than 5000A. Most are in excess of 10kA. None are close. When I look at the charts accompanying the curves, there really isn't going to be a solution there.

Just for grins, I tried artificially increasing the feeder length from 50' on up to see what it would take to knock down the available at the CP. It was a mere 1,350 feet. Probably not feasible, lol.

Do any of you have a suggestion if the CP manufacturer comes back with a "NO" answer as far as a combination rating?

Thanks.

One thought I had this morning would be to add in an isolation transformer. I can get very close to where it needs to be depending on the %Z. I suggested that to the person that asked for help.

Any other thoughts?

A current limiting reactor. Their construction is quite simple (air coil) so shouldn't be as expensive as a transformer.

Hmmm. That's a thought. I'll look into it to see if they can get where they need to. Google says Eaton, among others, has a product made for that. Thanks!

bbaumer I've done this a couple times with the air core current limiting reactor and many times with isolation transformers. In my experience, if you are worried about arc flash hazards, you also must install a fusible disconnect switch. The transformer will do its job to drop the fault current which will then greatly increase the arc flash hazard at the control panel. The addition of the fuse will remediate this. You can look into the Breaker Integrated Transformers (BIT) but last I checked they did not offer these for isolation transformers.

Got it. I think that is the approach they are taking. It's not my project, or problem, just trying to help. They were hoping to get the available down to under 14K and use J fuses. Thanks for taking the time to respond. I appreciate it.

Now see that current limiting fuses do not solve all problems.
The only thing you hope for is the control panel manufacturer offers a design with a higher SCCR or you need to add impedance to your circuit.
I typically used isolation transformers because they could be adjusted to compensate for voltage drop that often occurs with plain reactors. My second most common solution was to redo the feed to the control panel by lengthening the run or raising the impedance through smalller conductors, fewer parallel paths, or even different raceway material.